The proposed project deals with the chemistry of the assembly of bacteriophage T4D. Major efforts are being devoted to determining which proteins are involved in the assembly of the tail plate of the virus particle. Of major interest is the fact that the enzyme dihydrofolate reductase and now thymidylate synthetase, both phage-induced enzymes, appear to be structural components of this part of the viral capsid. Efforts are also continuing to examine the role of the viral- induced pteroyl hexaglutamate in the assembly of this part of the viral structure. A series of experiments using broken cell extracts has demonstrated that this folic acid derivative plays a critical role in viral assembly. A viral gene involved in its formation, gene 28, has been identified and each of its actions partially characterized.